1. Field of the Invention
This invention is directed to an image forming apparatus for use in copiers, printers and the like. More particularly, this invention is directed to a reversal development method type image forming apparatus.
2. Description of the Related Art
Traditionally, most image forming apparatus used in printers, copiers, facsimile machines and the like operate use the reverse development method. A typical photosensitive drum used in conjunction with the reversal development method is shown in FIGS. 11 and 12. FIG. 11 graphically represents the electrostatic potentials used in the reversal development method where a photosensitive drum potential Vo is placed on a photosensitive drum 141 and a developing sleeve potential Vb is placed on a developing sleeve 146.
FIG. 12 is a schematic view of the conventional photosensitive drum 141. A charger 144 charges the photosensitive drum 141 uniformly to 700 V. A power supply (not shown) charges the developing sleeve 146 to 550 V. Negatively charged carriers 154 and positively charged toner particles 152 attached to the carriers 154 are electrostatically attracted to the developing sleeve 146. The carriers 154 and the toner particles 152 rotate together with the developing sleeve 146. To print, a laser beam 150 is focused or directed onto the photosensitive drum 141 to selectively lower the previously charged 700 V potential down to a discharged 150 V potential. That is, the original 700 V potential is removed or discharged.
When the discharged portions of the drum 141 come into contact with the developing sleeve 146, the toner particles 152, which are attached to the carriers 154 attached to the developing sleeve 146, migrate to the discharged portions of the photosensitive drum 141 due to the 400 V potential difference between the drum 141 and the developing sleeve 146. That is, the discharged portion of the photosensitive drum 141 have a potential of 150 V, while the developing sleeve 146 has a potential of 550 V. To print the output image, the toner particles 152 attached to the photosensitive drum 141 are transferred onto recording paper (not shown) by a transfer roller 142.
When this conventional image forming apparatus is deactivated, a light beam from either an erase lamp (not shown) or the laser beam 150 is focused or directed onto the photosensitive drum 141 while the drum 141 is rotated, so that the drum surface potential is lowered or discharged to 150 V. The photosensitive drum 141 is rotated until the discharged portions come into contact with the developing sleeve 146. The potential applied to the developing sleeve 146 is then removed.
One disadvantage of the reversal development method, as outlined above, is its tendency to permit some of the toner particles 152 or the carriers 154 to migrate from the developing sleeve 146 to the photosensitive drum 141 when the image forming apparatus is stopped. FIG. 13 graphically represents the photosensitive drum potential Vo and developing sleeve potential Vb in effect when a typical conventional image forming apparatus is deactivated.
As illustrated, when the laser beam 150 is focused or directed onto the photosensitive drum 141, the potential Vo at point D, where the laser beam originally strikes the photosensitive drum 141, is immediately lowered to 150 V. On the other hand, when the power is removed from the developing sleeve 146, the potential on the developing sleeve 146 is rapidly reduced, as indicated by curve c, but not as rapidly as is the potential Vo from the photosensitive drum 141. This occurs because the developing sleeve 146 is equipped with a capacitor (not shown), which is used to prevent sparks. In such a setup, a large potential difference momentarily develops between the photosensitive drum 141 and the developing sleeve 146.
In particular, the large momentary potential difference occurs due to manufacturing errors that vary the distance between the laser beam-irradiated position D on the photosensitive drum 141 and the contact position, point E, between the photosensitive drum 141 and the developing sleeve 146. Hence, the time required for the point D, which has been irradiated with the laser beam, to reach the contact position, point E, between the photosensitive drum 141 and the developing sleeve 146 as the photosensitive drum 141 is rotated, is highly variable or unstable.
Thus, when the potential on the developing sleeve 146 is removed, based on a preferred time the laser beam-irradiated point D takes to reach the contact position, point E, deactivation of the developing sleeve potential Vb may occur before or after the laser beam-irradiated point D has arrived at point E. That is, the deactivation timing can be advanced or delayed relative to the correct timing. Thus, a timing mismatch can occur.
The deactivation of the developing sleeve potential Vb may be delayed beyond the laser beam-irradiated point D, as shown by curve b in FIG. 13. In this case, the potential Vo of the photosensitive drum 141 drops to 150 V at point D in FIG. 13, whereas the potential Vb of the developing sleeve 146 remains close to 550 V. This causes the positively charged toner particles 152 to stick to the photosensitive drum 146. With the toner particles 152 attached to the photosensitive drum 141 when the drum 141 is stopped, the toner particles 152 migrate to the transfer roller 142. When the next print operation is started, the toner particles 152 which migrated to the transfer roller 142 are transferred onto the back of the recording paper.
Conversely, the deactivation of the developing sleeve potential Vb may occur before the laser beam-irradiated point D arrives, as shown by curve a in FIG. 13. In this case, the potential Vo of the photosensitive drum 141 remains at 700 V, whereas the potential Vb of the developing sleeve 146 drops close to 150 V, as illustratively at point D in FIG. 13. This causes the negatively charged carriers 154 to stick to the photosensitive drum 141. Additionally, if oppositely charged toner particles 152 exist, in this case negatively charged toner particles 152, the oppositely charged toner particles 152 will also stick to the photosensitive drum 141 along with the carriers 154. In the following description, for illustrative purposes, only the carriers 154 are referred to. With the carriers 154 stuck to the photosensitive drum 141, these carriers 154 come between the transfer roller 142 and the photosensitive drum 141, as well as between a cleaning blade (not shown) and the photosensitive drum 141. The carriers 154, which are made of hard ferrite particles and the like, can damage the photosensitive drum 141, whose soft surface is, for example, formed by an organic photosensitive material.